My invention relates to a technique for providing an ac plasma panel with self-shift capability.
A plasma panel is a display device comprised of a body of ionizable gas sealed within a nonconductive, transparent envelope. Alphanumerics, pictures, and other graphical data are displayed by controllably initiating glow discharges (also referred to as "gas discharges") at selected locations within the display gas. This is accomplished by setting up electric fields within the gas via appropriately arranged electrodes, or conductors.
The invention principally relates to so-called twin-substrate ac plasma panels which have the conductors embedded within dielectric layers disposed on two opposing nonconductive surfaces, such as glass plates. Typically, the conductors are arranged in rows on one plate and columns orthogonal thereto on the other plate. The overlappings, or crosspoints, of the row and column conductors define a matrix of discharge cells, or sites. Glow discharges are initiated at selected crosspoints under the control of, for example, a digital computer. The computer initiates a discharge at a selected site via a "write" pulse which is impressed across (applied to) the site by way of its row and column conductor pair. The magnitude of the write pulse exceeds the breakdown voltage of the gas, and a plasma, or "space charge cloud," of electrons and positive ions is created in the crosspoint region. Concomitant avalanche multiplication creates the glow discharge and an accompanying short, e.g., one microsecond, light pulse in the visible spectrum. The write pulse, which continues to be impressed across the site, pulls at least some of the space charge electrons and ions, or charge carriers, to opposite cell walls, i.e., opposing dielectric surfaces in the crosspoint region. When the write pulse terminates, a "wall" voltage resulting from these so-called wall charges remains stored across the gas at the crosspoint.
A single short-duration light pulse cannot, of course, be detected by the human eye. In order to provide a plasma discharge site with the appearance of being continuously light-emitting (ON, energized), further rapidly successive light pulses are needed. These are generated by a sustain signal which is impressed across each site of the panel. The sustain signal is conventionally comprised of a train of alternating-polarity pulses. The magnitude of these sustain pulses is less than the gas breakdown voltage. Thus, the voltage across sites not previously energized by a write pulse is insufficient to cause a discharge and those sites remain in non-light-emitting states.
The voltage across the gas of a site which has received a write pulse, however, comprises the superposition of the sustain signal voltage with the wall voltage previously stored at that site. Conventionally, the sustain pulse which follows a write pulse has a polarity opposite thereto so that the wall and sustain voltages combine additively across the gas. This combined voltage exceeds the gas breakdown voltage and a second glow discharge and accompanying light pulse are created. The flow of carriers establishes an opposite wall voltage polarity. The polarity of the next sustain pulse is also opposite to that of its predecessor, creating yet another discharge, and so forth. After several sustain cycles, the magnitude of the wall voltage is established at a nominally constant, characteristic level which is a function of the gas composition, panel geometry, sustain voltage level, and other parameters. The sustain signal frequency is typically on the order of 40-50 kHz so that the light pulses emitted by an ON site in response to the sustain signal are fused by the eye of the viewer, and the site appears to be continuously light-emitting.
A site which has been established in a light-emitting state is switched to a non-light-emitting (OFF, de-energized) state via the application of an "erase" pulse thereto. The erase pulse creates one last discharge but removes the stored wall charge.
In the past, write and other pulses have been impressed across a selected gas discharge site principally by utilizing so-called half-select techniques in which opposite-polarity signals, each of nominally half the pulse magnitude are applied to the row and column conductors, respectively, of the site in question. These half-select signals are, of course, also thereby extended to each other site in the row and column of the selected site. Since they combine only across the selected site, however, only that site receives a full magnitude pulse and only that site responds thereto.
Disadvantageously, half-select writing and erasing requires an individual driver circuit for each row conductor and each column conductor. Each driver circuit, in turn, is typically comprised of a number of active and passive components. Since a plasma panel may have, for example, 512 row conductors and an equal number of column conductors, the requirement of a driver for each conductor substantially increases the cost, complexity and bulk of the display panel. Accordingly, numerous arrangements have been proposed to minimize the amount of circuitry required to drive an ac plasma panel. Among these are so-called self-shift displays in which the display information for each site in a given row, for example, is entered at one end of the row and is thereafter shifted to the proper column location by applying specially adapted shifting voltage waveforms to the column conductors. Typically, every third or fourth column conductor is connected to a common bus (depending on the specific shifting technique employed) so that only four or five column drivers are required--one for writing and three or four for shifting. Unfortunately, however, the self-shift arrangements known in the art typically suffer from one or more significant drawbacks, including severe signal margin requirements, low shifting speed, poor resolution, limited viewing angle and complex, expensive panel structure.